The present invention relates to a process for producing light olefins rich in ethylene and propylene from methanol and dimethyl ether.
A remarkable growth in the production of synthetic fibers, plastics and rubber has taken place in recent decades. This growth has, to a very large extent, been supported and encouraged by an expanding supply of inexpensive petrochemical raw materials such as ethylene, propylene, and other, four and five carbon olefins. Side by side with this growth, there has been an increasing demand for alkylate, made by reacting olefins with isobutane, for use as a high octane gasoline component.
Burgeoning demand for olefins, particularly ethylene, propylene and butenes, has of course led to periods of shortage, which has led to substantial price increases in the feedstocks to the commercialized technologies. These feedstocks are largely C2 to C4 olefins co-produced with natural gas and/or paraffinic straight run naphtha. These feedstocks tend to be substantially more expensive than methane, making it desirable to provide efficient means for converting methane to olefins.
Methane is an abundant low-value petrochemical feedstock. Methane is a less-expensive raw material than the ethane, LPG, and naphtha feedstock used today for the manufacture of ethylene and propylene. Although less expensive, methane is currently not used to produce light olefins because the manufacturing steps required are more expensive and/or less selective than the commercially utilized routes.
Conversion of methane to methanol, followed by conversion of methanol to ethylene and propylene appears to be the most attractive route to light olefins from methane. In this respect, it is known that methanol or dimethyl ether can be catalytically converted to olefin-containing hydrocarbon mixtures by contact under certain conditions with particular types of crystalline zeolite materials. For example, U.S. Pat. Nos. 4,025,575 and 4,038,889 both disclose processes whereby methanol and/or methyl ether can be converted to an olefin-containing product over a Constraint Index 1-12 zeolite catalyst, particularly ZSM-5. ZSM-5, in fact, converts methanol and/or dimethyl ether to hydrocarbons containing a relatively high concentration of light olefins with prolonged catalyst lifetime before catalyst regeneration becomes necessary.
It has also been reported that other types of zeolite catalysts can be used to convert methanol and/or dimethyl ether to olefin-containing hydrocarbons products containing even higher proportions of light olefins than obtained with ZSM-5. For example, U.S. Pat. No. 4,079,095 discloses that zeolites of the erionite-offretite-chabazite type, and especially ZSM-34, can usefully be employed to promote conversion of methanol and/or dimethyl ether to products comprising a major amount of ethylene and propylene. However, while enionite-offretite-chabazite type catalysts are highly selective to light olefins production, such smaller pore zeolites tend to age rapidly in comparison to ZSM-5 when used for methanolidimethyl ether conversion.
U.S. Pat. Nos. 4,677,242 and 4,752,651 disclose the conversion of methanol to C2-C4 olefins over various silicoaluminophosphates and xe2x80x9cnon-zeolitic molecular sievesxe2x80x9d (such as metal aluminophosphates) and teach that the addition of diluents, such as aromatic materials, having a kinetic diameter greater than the pore size of the molecular sieve increases the ethylene to propylene ratio in the product.
U.S. Pat. No 4,499,314 discloses that the addition of various promoters, including aromatic compounds, such as toluene, accelerates the conversion of methanol to hydrocarbons over zeolites, such as ZSM-5, which have a pore size sufficient to permit sorption and diffusion of the promoter. In particular, the U.S. Pat. No. 4,499,314 teaches that the increased conversion resulting from the addition of the promoter allows the use of lower severity conditions, particularly lower temperatures, to increase the yield of lower olefins (column 4, lines 17-22).
In contrast, U.S. Pat. No.6,046,372 discloses that the addition of an aromatic cofeed allows the selective conversion of methanol to C2 to C4 olefins at relatively high temperatures (350 to 480xc2x0 C.) and relatively high methanol partial pressures (15 to 150 psia) over a zeolite having a pore size greater than the critical diameter of the cofed aromatic compound.
According to the invention, it has now been found that improved ethylene and propylene selectivities can be achieved in the conversion of methanol by using the following combination of conditions: a zeolite catalyst with limited diffusivity, a cofed aromatic compound; high temperature; and a methanol partial pressure less than 50 psia. Methanol partial pressure can be reduced by a number of well known means, for example, by the addition of a diluent such as steam.
The present invention resides in a process for converting methanol and/or dimethyl ether to a product containing C2 to C4 olefins which comprises the step of contacting a feed which contains methanol and/or dimethyl ether with a catalyst comprising a porous crystalline material, said contacting step being conducted in the presence of a cofed aromatic compound under conversion conditions including a temperature of about 350xc2x0 C. to about 550xc2x0 C. and a methanol and/or dimethyl ether partial pressure less than or equal to 50 psia (345 kPa), said porous crystalline material having a pore size greater than the critical diameter of the aromatic compound and having a Diffusion Parameter for 2,2-dimethylbutane of about 0.1 to about 26 secxe2x88x921 when measured at a temperature of 120xc2x0 C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa), and the aromatic compound being capable of alkylation by the methanol and/or dimethyl ether under said conversion conditions.
Preferably, the molar ratio of methanol and/or dimethyl ether to aromatic compound is from 0.5:1 to 100:1, and more preferably from about 3:1 to about 30:1.
Preferably, the conversion conditions include a temperature of about 400xc2x0 C. to about 500xc2x0 C.
Preferably, the conversion conditions are such that the methanol conversion rate is less than 90% and more preferably less than 80%.
Preferably, the porous crystalline material has a pore size between 5 and 7 Angstrom.
Preferably, the porous crystalline material is an aluminosilicate zeolite and most preferably is ZSM-5.
Preferably, the porous crystalline material has a Diffusion Parameter for 2,2-dimethylbutane of about 0.1 to 15 secxe2x88x921, and more preferably 0.2 to about 5 secxe2x88x921, when measured at a temperature of 120xc2x0 C. and a 2,2-dimethylbutane pressure of 60 torr (8 kPa).
Preferably, the catalyst has an alpha value less than 10 and more preferably less than 2.